Recent Progress in Silicon-Based MEMS Field Emission Thrusters

Abstract

The Indium Field Emission Thruster (In‐FET) is a highly characterized and space‐proven device based on space‐qualified liquid metal ion sources. There is also extensive experience with liquid metal ion sources for high‐brightness semiconductor fabrications and inspection Like gridded ion engines, In‐FETs efficiently accelerate ions through a series of high voltage electrodes. Instead of a plasma discharge to generate ions, which generates a mixture of singly and doubly charged ions as well as neutrals, indium metal is melted (157°C) and fed to the tip of a capillary tube where very high local electric fields perform more‐efficient field emission ionization, providing nearly 100% singly charged species. In‐FETs do not have the associated losses or lifetime concerns of a magnetically confined discharge and hollow cathode in ion thrusters. For In‐FETs, propellant efficiencies ∼100% stipulate single‐emitter currents ⩽10μA, perhaps as low as 5μA of current. This low emitter current results in ⩽0.5 W/emitter. Consequently, if the In‐FET is to be used for future Human and Robotic missions under President Bush’s Exploration plan, a mechanism to generate very high power levels is necessary. Efficient high‐power operation requires many emitter/extractor pairs. Conventional fabrication techniques allow 1–10 emitters in a single module, with pain‐staking precision required. Properly designed and fabricated In‐FETs possess electric‐to‐jet efficiency >90% and a specific mass <0.25 kg/kWe. MEMS techniques allow reliable batch processing with ∼160,000 emitters in a 10×10‐cm array. Developing a 1.5kW 10×10‐cm module is a necessary stepping‐stone for >500 kWe systems where groups of 9 or 16 modules, with a single PPU/feed system, form the building blocks for even higher‐power exploration systems. In 2003, SNL and ARCS produced a MEMS‐based In‐FET 5×5 emitter module with individually addressable emitter/extractor pairs on a 15×15mm wafer. The first MEMS thruster prototype has already been tested to demonstrate the proof‐of‐concept in laboratory‐scale testing. In this paper we discuss progress that has been achieved in the past year on fabricating silicon‐based MEMS In‐FETs.